In recent years, there has been increasing interest in quantum communication. The basis for quantum communication is the creation of quantum particles (most often photons) in what is known as entangled states. Two particles are in an entangled state if measurements of their properties (e.g., time of arrival, energy, spin, polarization, etc.) are statistically correlated. One simple example is spin correlation, where if particles A and B are entangled with opposite spin, then spin measurements of A and B tend to produce opposite results (i.e., the measured spins of A and B are correlated).
It is important to note that these quantum correlations can be measured even if the measurements on A and B are performed at locations separated arbitrarily far away from each other. Thus the presence of entangled state correlations in such cases demonstrates a peculiar quantum non-locality. The exploitation of such quantum non-locality for practical purposes is the main focus of quantum communication.
Quantum communication is thus seen to rely on a source of particle pairs in suitable entangled states. One source of entangled photon pairs (often referred to as biphotons) that has been investigated is spontaneous parametric down conversion (SPDC), where a pump photon splits into a signal photon and an idler photon in a nonlinear optical medium. The signal and idler photons are entangled. However, practical application of SPDC for producing biphotons has been hampered by several factors, including excessive biphoton line width and low source brightness. Accordingly, it would be an advance in the art to alleviate these issues.